A wide variety of systems are controlled by voltage variations, for example, a switch-operated automatic door system. Another example is a system wherein a process parameter actuates a switch to activate a subsequent process step or sound an alarm. In such systems, a control voltage is impressed upon the switch contacts such that when the switch is closed, the control voltage is applied to system components in series with the switch. Ideally, the contact resistance in the switch is infinite when the switch circuit is open and substantially zero when the switch contacts are closed. The voltage across the switch in the open position should be essentially equal to the control voltage, and the voltage across the switch in the closed position should be zero.
Any number of factors can cause the ideal characteristics of the system switch to deteriorate. Environmental effects, for example, may increase the contact resistance in the switch in the closed position and/or decrease the contact resistance in the open position. Such deterioration necessarily operates to decrease the difference in the voltage across the switch in the open and closed positions.
Contact resistance deterioration can lead to unsafe operating conditions in such systems. For example, in an automatic door system a control mat is placed in the doorway to actuate a door control unit to open the door. A safety mat is disposed after the door to hold the door open while a person is passing through the doorway. If the switch under the safety mat has deteriorated or become inoperative, the door may close while the person is on the safety mat and thereby cause injury to the person. By way of further example, in an industrial process where a switch is employed to trigger a safety alarm, as the switch contacts reach a certain degree of deterioration, the alarm will not be activated when the dangerous condition is sensed.
It can therefore be seen that there exists a need in many environments for a monitor for sensing voltage variations. Such a monitor could, for example, monitor the contact resistance across a switch, and disable the switch-controlled system when the variation in switch resistance between the open and closed positions of the switch exceeds an acceptable limit.